FIG. 1 illustrates a conventional frontside illuminated complementary metal-oxide-semiconductor (“CMOS”) imaging pixel 100. The frontside of imaging pixel 100 is the side of substrate 105 upon which the pixel circuitry is disposed and over which metal stack 110 for redistributing signals is formed. The metal layers (e.g., metal layer M1 and M2) are patterned in such a manner as to create an optical passage through which light incident on the frontside of imaging pixel 100 can reach the photosensitive or photodiode (“PD”) region 115. The frontside may further include a color filter layer to implement a color sensor and a microlens to focus the light onto PD region 115.
Imaging pixel 100 includes pixel circuitry disposed within pixel circuitry region 125 adjacent to PD region 115. This pixel circuitry provides a variety of functionality for regular operation of imaging pixel 100. For example, pixel circuitry region 125 may include circuitry to commence acquisition of an image charge within PD region 115, to reset the image charge accumulated within PD region 115 to ready imaging pixel 100 for the next image, or to transfer out the image data acquired by imaging pixel 100. As illustrated, in a frontside illuminated configuration, pixel circuitry region 125 is positioned immediately adjacent to PD region 115. Consequently, pixel circuitry region 125 consumes valuable real estate within imaging pixel 100 at the expense of PD region 115. Reducing the size of PD region 115 to accommodate the pixel circuitry reduces the fill factor of imaging pixel 100 thereby reducing the amount of pixel area that is sensitive to light, and reducing low light performance.